Filament wound composites are frequently used in industries where large high strength tubular structures are required. For example, filament wound composites are used in the defense industry to make missile capsules and torpedo tube launch capsules. These composites are also used to make large pipes for transporting liquid such as oil and well water. The primary advantages in using filament wound composite materials is that they can be inexpensively produced, and the composites have exceptionally high strength and stiffness characteristics and are non-corrosive.
The filament winding process is based on high-speed precise lay-down of filaments, usually glass or graphite, in predescribed patterns. The process involves winding continuous resin-impregnated rovings or bands, i.e., gathered strands of filaments, over a male mandrel. The mandrel can be cylindrical, round, or any shape that does not have re-entrant curvature. The bands may be wrapped either in adjacent bands or in repeating bands that are stepped the width of the band and which eventually cover the mandrel surface. The technique has the capacity to vary the winding tension, wind angle, or resin content in each layer of reinforcement until the desired thickness and resin content of the composite are obtained with the required direction of strength.
One of the problems associated with the filament winding technology is how to join two tubular composites. Numerous types of composite joints have been designed, most of which involve some type of male-female connection, usually increasing the overall joint thickness in the joint area. For example, the bell and spigot joint involves an enlarged female composite end adapted to receive a male composite end. The result is a joint having an enlarged outer diameter. Variations of the bell and spigot joint include the mechanical joint, the threaded and bonded joint and the O-ring joint. All of these joints are relatively thick compared to the rest of the composite, making such joints unsuitable for applications which have tight stacking or storage requirements.
Thus, it is advantageous to produce a shallow tapered scarf joint. Such a joint provides a connection between two thin wall composite structures without the need for intervening metal or other material sections. The joint is advantageous in that it transfers loads gradually, without abrupt metal-to-composite transitions, which can cause stress concentrations or a discontinuity in coefficients of thermal expansion. Further, in a tapered scarf joint the composite thickness is constant between the composites and throughout the joint.
Tapered scarf joints have been used in the aircraft and aerospace industries to join composite structures. However, they have not been used with filament wound composites. The reason is that there have been only two cost-prohibitive ways to produce such a joint which has both hoop and helical fibers. One way to produce a tapered scarf joint is by manually machining the two composite ends. The other way is to use a 6-axis numerically controlled filament winding machine which requires elaborate computer control means for precisely cutting successive composite plies to variable lengths. Only a limited number of these machines are in existence, and they are exceptionally expensive. In contrast, conventional filament winding machines are relatively common and inexpensive.
Therefore, it is an object of the present invention to produce a high strength relatively inexpensive joint between two filament wound composite tubes, by modifying a conventional filament winding machine.
Another object is to produce such a joint without inserting additional parts into the joint, and without increasing either the inner or outer diameters of the tubes.